The study by two veteran defense analysts working for the Washington Institute for Near East Policy also said the US operation in Iraq was at a "tipping point" that will last for six to nine months.

"I think the outcome of this tipping period is probably going to dictate whether or not the US effort in Iraq succeeds or fails," analyst Jeffrey White said at a lunch unveiling the report.

The study said the insurgency, comprising of nationalists, members of Saddam Hussein's toppled regime and foreign Islamic fighters, showed no sign of losing steam 32 months after the US-led invasion.

"Although thousands of insurgents have been killed and tens of thousands of Iraqis have been detained ... incident and casualty data reinforce the impression that the insurgency is as robust and lethal as ever," it said.